RF Digital Wattmeter Basics

RF wattmeters are devices that measure power (watts) into a circuit or system such as a transmission tower. Bi-directional wattmeters allow the user to take data on a system while it is operating. In addition to delivered power, they also measure reflected power. Digital RF wattmeters can be used to make more advanced measurements, because they are able to digitize instantaneous power at many points along a pulsed signal. RF modulation schemes such LTE, QAM, OFDM, AIS, FM, and AM can all be analyzed by a digital wattmeter. The meter can be used to extract parameters such as: Forward Power (Peak, Average, and Crest Factor) and Reflected Power (VSWR and/or Return Loss)

The digital wattmeter must sample the instantaneous power, and be able to synchronize to the given modulation scheme. The sensor(s) on a digital power meter must react quickly to power levels of RF signals. There are many choices for sensors, including bolometers, thermo-couples and diode detectors. Bolometers use thermistors that have steep resistance slope versus temperature; the speed of a bolometer depends on how small the thermal mass of the thermistors can be made. Typically, bolometers are not fast enough for high-speed measurements. Thermocouples use the Seebeck effect to generate a voltage from two junctions, one heated by the RF signal and the other held at a constant (lower) temperature. Diode detectors rectify RF waveforms to produce a voltage that is a good representation of the RF envelope. RF digital wattmeters typically employ diode detectors or thermocouples.

Analog versus Digital Considerations

Deciding to use one type over the other will be influenced by many factors, such as the environment, the application, and the ultimate intended use of the measured data. Some basic differences between the two types of wattmeters include:

Display

Analog meters display results by moving a needle across a scale, and because of the slow movement of the display are restricted to reporting only average values of power. Digital power meters can have elaborate displays that show RF waveforms, and can also record digitized data. When tuning a circuit, it is easier to watch an analog meter’s needle move as an indication of change in reflected power in an analog meter, versus concentrating on a digital readout. However, the digital display makes it easier to see smaller incremental changes.

Additional calculations

An analog wattmeter displays only forward and reflected average power levels. Critical parameters such as voltage standing wave (VSWR) or return loss can be manually computed by the user. A digital wattmeter will calculate and display average and peak measurements, provide a graphic of pulsed power for a modulated waveform, and allow direct measurements of nonlinear parameters such as Crest Factor.

External power needs

A digital wattmeter requires some form of external power to run the computer chip that performs the calculations and to maintain the internal temperature. There is typically either a power cord, battery, or some sort of rechargeable power supply. Modern digital wattmeters have rechargeable battery packs that can last up to 8 hours per charge to allow use in the field.

Accuracy

Digital meters provide higher accuracy, displaying and storing exact measurements. Analog meters rely on the individual operator’s interpretation of the position of the needle at a given point, although experienced users using the right scale can generate more than sufficient accuracy for many applications.